In underground mining the hydraulic operations, usually effected via hydraulic cylinders or hydraulic props, are initiated by a number of electromagnetic actuators for switching electrohydraulic valves for the hydraulic cylinders. The large number of electrohydraulic valves for switching, for example in powered shield-type walking supports, place especially exacting requirements on the power supplies and the available underground energy supply system, and furthermore both the power supply and the energy supply systems must be intrinsically safe and meet all the requirements regarding protection from explosions and/or other forms of ignition. Since this involves limiting the installed electric loads made available by the power supply, it is conventional in underground mining to provide the electromagnetic actuators with holding-current lowering means. Also the large inductive loads in the intrinsically safe energy supply systems make it necessary to take special precautions to ensure conformity with the anti-ignition regulations.
One known problem in the case of electromagnetic actuators with energised coils is the residual energy pulse after cutting off the inductive load. As a counter-measure in existing underground mining, all electromagnetic actuators are provided with at least two short-circuit means usually in the form of free-wheeling diodes, which short-circuit the coil in the event of a reversal of potential of the coil voltage, in order in all cases to avoid voltage peaks of the cut-off energy or of the residual energy pulse. The magnetic field, which collapses when the coil is switched off, induces a voltage in the coil, with the opposite polarity from the connected voltage, and this polarity change, hereinafter referred to by the general term “potential reversal”, unlocks the freewheeling diode so that a current flows through it for dissipating the magnetic field of the coil.
However, in underground energy supply systems comprising a number of simultaneously switched electromagnetic actuators, the residual cut-off energy pulse resulting for example from a broken cable or the like, even if reduced by freewheeling diodes, will result in a total pulse beyond that permitted by the anti-ignition regulations.
An aim of the invention is to devise a protective circuit for electromagnetic actuators or for intrinsically safe underground energy supply systems wherein even under the most unfavourable conditions such as a broken cable or power failure, there is no possibility of an overall pulse resulting in gas ignition or other operating states not permitted by the anti-ignition regulations.